1. Field of the Invention
The present invention relates generally to multiple power converters used in conjunction, and relates more particularly to a system and method for managing power converters used in conjunction as paralleled phases of a multiphase switching power supply.
2. Description of Related Art
Power supplies are often paralleled to meet particular application goals, such as current or power specifications. Performance improvements in interleaved, multiphase power supplies can be seen from advantages such as reduced input current ripple, reduced peak output current and greater frequency output ripple current. The higher frequency output ripple current permits easier filtering of the output ripple current to remove the ripple. Multiple interleaved phases in switching power supplies also tends to improve power conversion efficiency. A particular type of multiphase switching power supply has a variable switching frequency to obtain desired power supply output characteristics.
A variable frequency switching power supply may operate in various modes at various times, depending upon desired characteristics. For example, a switching power supply may operate in continuous, discontinuous or transition mode, each of which have various advantages. A switching power supply may be constructed to have an inductor that is supplied with current for a given interval and permitted to discharge to a certain extent. Such a switching power supply operating in a continuous mode permits an inductor to discharge to a point where the inductor current is still positive, or above zero, before charging the inductor again. A discontinuous mode switching power supply permits the current in the inductor to drop and remain at zero for a finite time before charging the output inductor again in a subsequent switching cycle. A transition mode switching power supply permits the inductor to discharge to zero current, at which point a new charging cycle begins, so that the inductor current is prevented from becoming negative or remaining zero.
One advantage to transition mode operation is the potential for zero voltage and/or zero current switching in the power supply. Zero voltage switching and zero current switching permits switching losses to be reduced, which can be especially advantageous at high frequencies that are often seen at light loads.
Another advantage to transition mode operation is that it provides a simple way to maintain a desired power factor for a power converter. A typical transition mode configuration for a power converter permits the current in the inductor to achieve a peak value that is proportional to the input voltage. The momentary average of the current through the inductor is proportional to the instantaneous value of the input voltage, which permits the power converter to draw power from an input source at unity power factor. It is desirable to maintain the power factor as close as possible to unity, so that the power converter appears as a purely resistive load on the input power line. Factors that contribute to improving the power factor include maintaining input voltage in phase with input current, and maintaining the input current as a sinusoid when the input voltage is a sinusoid. Transition mode operation tends to help support realization of a good power factor in a variable frequency switching power supply.
A variable frequency transition mode power converter constructed with an inductor can be viewed as a free running oscillator with the switching frequency being controlled by the amplitude of the inductor current. As the load demand decreases, the switching frequency tends to increase as inductor current amplitude decreases. Two or more transition mode power converters may be paralleled to obtain desired operating characteristics, such as a desired output current or power level. The paralleled power converters may also be interleaved and their waveforms synchronized to obtain the advantages discussed above. As switching frequency increases in a paralleled, interleaved power converter during light load conditions, the efficiency of the power converter decreases substantially. The switching losses experienced by the paralleled, interleaved power converters during high frequency switching tend to dominate converter losses over conduction losses. A number of applications for paralleled, or multiphase power converters have loads that can vary significantly, with light load demand extending over relatively long periods of time. It would be desirable to improve the efficiency of multiphase power converters during light load demand intervals.